Upgrade to atom-shell@0.19.0

@kevinsawicki From atom-shell v0.19.0 all custom security restrictions on `iframe` have been dropped, and package authors need to use [`<webview>` tag](https://github.com/atom/atom-shell/blob/master/docs/api/web-view-tag.md) for embedded external contents instead, we may need to put a notice in Atom's release notes.